As known, a yarn feeder for textile/knitting lines typically comprises a stationary drum on which a motorized flywheel winds a plurality of yarn loops forming a well reserve. Upon request from a downstream machine, e.g., a loom, the loops are unwound from the drum and, before reaching the machine, the yarn passes through a weft-braking device that influences the tension of the unwinding yarn.
A typical weft-braking device may comprise a hollow, frustoconical braking member, which is biased with its inner surface against the delivery end of the stationary drum in order to pinch the unwinding yarn and brake it by friction.
A weft-braking device of the above-mentioned type can apply a static, adjustable braking action to the yarn or, according to the teachings of EP 1717181 B1 of Applicant, it can be operatively connected to electronically controlled driving means capable of applying a modulated braking action which maintains the yarn tension at a desired level, in order to reduce the risk of yarn breakage, to prevent defects in the finished products, and to optimize the production yield.
In more detail, in EP 1717181 B1 the frustoconical member is supported by a spider-assembly of springs which have one end connected to the smaller end of the frustoconical member and the opposite end connected to an annular support. The annular support, in turn, is supported at two diametrically opposite positions by the operative rods of two linear electromagnetic actuators attached to the body of the yarn feeder and acting in directions parallel to the axis of the drum. The electromagnetic actuators are driven by a position control loop to modulate the action of the frustoconical member against the drum, as mentioned above.
An advantage of the above-mentioned braking system is that it does not require frequent cleaning operations because the dust and paraffine generated by the yarn running between the braking surfaces are swept away by the swivel movement of the yarn unwinding from the drum.
However, the above-mentioned device has the drawback that it is relatively complex—and therefore expensive—to manufacture both from the mechanical point of view and in relation to the dedicated power electronics required.
Also, the electromagnetic drive is not entirely satisfactory in terms of reaction times, because the excitation times of the coils are notoriously non-negligible and the movable masses involved are considerable, thereby resulting in a high inertia.
In addition, the electromagnetic drive requires high currents and, therefore, high power, with consequent disadvantages in terms of energy consumption, especially in view of the fact that a conventional textile/knitting line often makes use of dozens of feeders for a single downstream machine.